This invention relates to apparatus for and a continuous method of producing strip or sheet (hereinafter referred to simply as "strip") by a spray forming technique in which a plume or spray of metallic particles or droplets at elevated temperatures is deposited onto a suitably shaped receptor surface and removed therefrom in strip form.
In one particular method of spray forming, a stream of molten metal falling freely under gravity is atomised by a system of high pressure jets to form the required plume or spray of droplets which, when they impinge on the suitably shaped receptor under appropriate conditions build up to form a solid artefact for subsequent hot compaction as required.
Hitherto, it has been possible to produce sheets or plates of relatively thick section by such methods: however the correct conditions for the continuous production of near-to-final thickness strip have not been established.
Spray forming techniques can be used to produce a wide variety of metallic strips of different compositions. Such techniques do, however, have particular application to the continuous production of electrotechnical steels.
As is well known, the operation of transformers, motors, generators and like electrical machines depends upon the phenomenon of electromagnetic induction whereby current changes occurring in a (primary) coil are linked magnetically with a proximate (secondary) coil to cause a corresponding voltage to develop across the secondary winding, the value of which depends on the ratio of primary to secondary turns. The magnetic linkage effect is multiplied by many orders if the windings are formed upon a circuit of ferrous material so greatly enhancing the efficiency of the machine. As the current in the primary coil changes to establish a magnetic flux in the core, small currents called eddy or Foucault currents flow in the core material itself in a plane normal to the direction of the magnetic flux established in the core. Thus, if primary coil current is changing at, say, mains frequency (50 Hz), these eddy currents cause heating of the core material which is electrically conducting. Such heating effects are related in magnitude to the second power of the exciting frequency and represent power lost to the machine system. Therefore every effort is made to reduce the eddy currents. Two approaches are employed viz
(a) make up the core from laminated sheet, each layer being typically 0.30 mm thick and both surfaces carrying a very thin (micron) layer of electrical insulant; PA1 (b) increase the intrinsic resistivity of the material itself.
We are aware, of course, that total power loss in a machine system has a second component, the hysteresis loss, and that this is dealt with by developing the grain structure of the core material so that in grain size and orientation magnetic domains of favourable form are engendered within each grain and thus improve the flux carrying capacity of the material with least loss of energy manifesting itself as wasteful heat.
To those skilled in the art, it has long been known that the addition of silicon and aluminium to iron produces a wide range of electrotechnical steel strip incorporating these features and from which machine cores can be assembled. However, even the best known conventional steelmaking techniques are limited in the amount of material such as silicon and/or aluminium which can be added to iron to increase the electrical specific resistance of the ensuing material or improve the grain structure and hence domain dynamics because the addition of such elements causes the material to become so brittle as to prevent working due to a coarse grain formation. The presence in the material of silicon/aluminium in quantities of above about 3.25% means that the material cannot be cold reduced without the onset of cracking. Cold reduction is required for economic production and also to develop desired properties in important grades of electrotechnical steels as well as to improve surface quality.
Attempts have been made to overcome these problems by the production of amorphous (non-crystalline) material by melt spinning processes but these materials are, to date, very thin as spun (about 40 micron max) and extremely brittle rendering them inappropriate for assembly into very large electrical machines.
Therefore, the application of the spray forming technique to the continuous production of electrotechnical steel strip presents itself as a possible means of producing material comparable in mechanical handling to conventional material but with enhanced resistivity and/or grain/domain structure due to the fact that the spray forming technique permits the addition of silicon/aluminium to levels far beyond those possible with conventional techniques while still retaining a small grain size or permits the formation of alloys not possible by conventional processing due to, for example, segregation.
It is an object of the present invention to provide apparatus for and methods of producing strip continuously by a strip forming process which at least alleviates disadvantages present in previous techniques. More especially, but not exclusively, it is an object of this invention to provide apparatus and methods by which electrotechnical steel strip of enhanced magnetic performance can be produced continuously by spray forming.
The invention also sets out to demonstrate other benefits which accrue from the use of the technique to be described such as re-use of scrap material, use of compositions (alloys) with interesting magnetic properties hitherto prohibited by phase diagram limitations etc.